It is known that the synthetic aperture radar, or SAR, is a microwave remote sensing active system that, installed onboard aircrafts or satellites, allows high resolution images of the observed scene to be generated independently from meteorological conditions and presence of sunlight. Generally, the wavelength of the microwaves used in a SAR system ranges from 3 to 30 cm.
Sensing resolution substantially depends on the band of the signal transmitted by the radar, the antenna beam angle and the distance between the radar system and the observed surface.
In applications requiring a continuous monitoring, as for instance in ground motion monitoring, the radar is preferably installed onboard a satellite. Notwithstanding the large distance at which such radar operates, equal to about 800 Km, SAR technology allows a high spatial resolution to be obtained, through an operation of processing the echo reflected by the surface irradiated by the transmitted signal. In particular, such processing makes a focusing of the image synthesising the reflected echo so as to simulate an antenna aperture significantly larger than that of the antenna actually installed onboard the aircraft.
One of the characteristics of the SAR systems is their capability to follow both the amplitude and the phase behaviour of the backscattered echo. In some applications, such as the Ground Motion Monitoring, the presence of Permanent Targets, or PTs, over the territory under observation allows possible ground variations to be detected through the analysis of the time sequences of the images obtained from the same scene. PTs have radio scattering properties known to the SAR system which remain stable in time.
Present PTs may be passive devices, such as reflectors known to the skilled in the art as “corner reflectors”, or antenna active transponders. In the latter case, the signal re-transmitted by the transponder must have the same frequency of the received signal and a phase relationship with the received signal that is stable in time.
German Patent Application No. DE 32 48 879 A1 discloses a system adopting the technique of identification of the signals transmitted by the transponder with respect to the neighbouring ones, through transmission power and time location.
Such system suffers from some drawbacks. In fact, it subtracts scattering zones, and it does not allow the object to be distinguished from other close ones of the same kind.
U.S. Pat. No. 5,821,895 discloses a system adopting the technique of identification of the signals from the transponder through encoding of the single pulse. The SAR receiver device carries out correlations along azimuth and range directions of the received signal with the transmitted and expected SAR signal. These correlations do not detect the presence of the transponder for low correlation because of the presence of the code on the signal generated by the transponder. Differently, if correlations take account of the encoding on the transponder signal, then they detect a correlation peak in correspondence with the transponder position. In this way, the signal transmitted by the transponder is scarcely visible by the SAR receiver and it has not to be delayed to minimum scattering instant of poor interest. The system of U.S. Pat. No. 5,821,895 also adopts the technique of detection of the single SAR pulse for applying the code to the signal received by the SAR device.
Even this system suffers from some drawbacks.
First of all, the adopted technique is subject to detection errors due to the noise of the transponder receiver. Single pulse detection errors are terrible because when a pulse is lost, the applied code is misaligned and correlation is compromised or annulled.
Moreover, the system of U.S. Pat. No. 5,821,895 adopts the technique of intermediate frequency modulation, hence requiring the use of synthesisers and frequency converters which introduce harmonic and spurious components which may jeopardise the signal processing and increase the background noise.